Conventionally, a capillary thermal-type mass flow rate sensor, or a silicon-made ultra-small sized thermal-type mass flow rate sensor, for which micro-machine technologies have been employed, has been widely used to measure a mass flow rate of fluid in various technologies, such as in chemical analysis equipment and the like. The former, or capillary thermal-type mass flow rate sensor, is characterized by the fact that the sensor has its gas contacting faces made of stainless steel due to its structure, which enhances the corrosion resistance of the sensor to fluids that it measures.
Also, the capillary thermal-type mass flow sensor must be equipped with a resistance wire for a heater that is wound to heat a capillary tube, which causes a problem that leads to unevenness in property among products. Another problem is that the response speed of the mass flow rate sensor is slow due to the relatively large heat capacities of the capillary tube and the resistance wire forming the heater.
On the other hand, along with development in recent years of the so-called “micro-machine” technologies, the development and utilization of the latter, or of a silicon-made ultra-small sized thermal-type mass flow rate sensor, have been widely under way. It has become popular, not only in the chemical-related fields, but also in industrial manufacturing fields such as the automobile industry and the like, because a silicon-made ultra-small sized thermal-type mass flow rate sensor can be manufactured by a single processing, thus reducing unevenness in properties among products. Furthermore, extremely fast response speed as a sensor can be achieved by making heat capacities small by downsizing the sensor, all of which are regarded as desired characteristics of a sensor.
However, it is noted that there exist many problems to be solved with silicon-made ultra-small sized thermal-type mass flow rate sensors. Among other things, corrosion resistance is one problem that is urgently needed to be solved. That is, a silicon-made ultra-small sized mass flow rate sensor employs silicon as a constituent component to form gas contacting faces. Therefore, a fundamental difficulty encountered with conventional sensors is that these silicone gas contacting faces can be easily corroded by fluids of the halogen family and the like.
Furthermore, organic materials such as epoxy resin, an O-ring and the like, are used as sealing materials for the mass flow rate sensor, thus making the emission of particles, or the occurrence of an outside leak, unavoidable. Consequently, such sensors cannot be employed for the gas supply line, and the like, in semiconductor manufacturing facilities.
Furthermore, another problem exits with the mass flow rate sensor, namely, fluctuation in detecting values of the mass flow rate sensor occurs when the pressure of the fluid to be measured changes, or there may be distortion of the sensor itself caused by a mechanical tightening force (or thrust) that occurs when the mass flow rate sensor is fitted to the gas supply line. These problems cause unevenness in detecting values of the mass flow rate sensor.
So far, various techniques have been developed to solve these difficulties with the afore-mentioned silicon-made ultra-small sized thermal-type mass flow rate sensor. For example, with the devices of TOKU-KAI No. 2001-141540 and the TOKU-KAI No. 2001-141541 and the like, there is provided a temperature resistance layer E6 on the outermost layer of a film E, formed on the upper face of the frame D made from a silicon substrate A, as shown in FIG. 20, to enhance stability of the film E. As shown in FIG. 20, E1 to E3 designate a silicon oxide layer forming a film E, E4 designates a silicon nitride layer, E5 designates a platinum layer, and C designates lead connecting hardware.
As stated above, with the afore-mentioned silicon-made ultra-small sized thermal-type mass flow rate sensor, as shown in FIG. 20, there is formed a silicon nitride layer E4 on the lower face side of the frame D, or a temperature resistant layer E6 consisting of a silicon nitride layer, to enhance water resistance and moisture resistance of the film E.
Patent Document 1: TOKU-KAI No. 2001-141540 Public Bulletin
Patent Document 2: TOKU-KAI No. 2001-141541 Public Bulletin